The time course of postnatal development of cardiovascular regulatory mechanisms is being studied in piglets Less than 1 day-3 months of age, from the viewpoint of efferent out-flow of the regulatory system. Because of the close interrelationship between the cardiovascular and respiratory controlling systems, aspects of developing postnatal regulation of the latter system are also being studied. Three specific aims are being pursued: 1) age-related differences in sympathetic activity (both efferent discharge and plasma catecholamine levels), 2) characteristics of development of autonomic regulation of the heart (recordings from cardiac vagal motoneurons and use of HRP techniques to locate cardiac sympathetic postganglionic soma), 3) age-related changes in the respiratory rhythm generator (efects of various forcing inputs, e.g., lung inflation, chemoreceptor stimulation). Recordings will be obtained from Althesin anesthetized or decerebrate unanesthetized piglets of: efferent sympathetic activity (splanchnic, cervical sympathetic, renal), efferent phrenic discharge, cardiac vagal fibers, motoneurons in nucleurs ambiguus and ventrolateral nucleus ambiguus, aortic pressure, EKG, regional arterial blood flow (particularly renal, mesenteric) and appropriate marking pulses. The age-related effects of alterations in afferent discharge from various cardiovascular-respiratory receptors, e.g. cardiopulmonary, arterial baroreceptors, on central cardiovascular and respiratory controlling systems will be evaluated. Common periodicities in sympathetic discharge and periodicities related to the cardiac cycle (indicated by EKG) or central respiratory cycle (indicated by efferent phrenic discharge) are being revealed by cross correlation and power spectral analyses, particularly tests for power spectral equivalence and coherence. The effects of brain stimulation and of stresses such as hypercapnia, hypoxia and hemorrhage will also be evaluated. Age-dependency of results will be determined statistically in 6 age groups of piglets. The results will contribute to our understanding of circulatory adjustments to extrauterine environment and to our knowledge of the degree of regulation of cardiovascular function available in the neonate. Information will be obtained which is necessary for the understanding of reflexes involved in physiological adjustments to various stresses (hypercapnia, hemorrhage, hypoxia), relevant especially to the most vulnerable period in the life of the child.